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Issue #211 February 2008
RFID Payment Terminal
by Carlos Cossio
Start | System Overview | Energy Transmission | 100% Amplitude Modulation | Load Modulation | MIFARE Card | MIFARE and ISO 14443 | Pay as You Go | Hardware Design | Antenna Design Rules | Antenna Size | Directly Matched Antenna | Firmware Design | Catch the Wave! | Sources & PDF
100% AMPLITUDE MODULATION
To transfer data from the reader to the card, the contactless card uses a half-duplex communication structure. The reader talks first and starts the communication. The data transmission from the reader to the card is done using a 100% ASK pulse-pause modulation according to ISO 14443 Type A.
Due to the quality factor Q of the antenna, the transmitted signal deforms. This shape can be used to measure the tuning of the antenna. Later, I will describe the theoretical background of calculating the antenna quality factor Q and explain how to calculate the components of the matching circuitry.
The contactless card is usually passive. To enable communication between the reader and the card, energy has to be transmitted to the card. Therefore, the contactless card uses optimized coding to provide a constant level of energy independently from the data transmitted to the card. This is the modified Miller code, which is used to transmit data from the reader to the card. Figure 2 shows Miller coding in detail.
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| Figure 2—The modified Miller coding provides a constant level of energy independently from the data transmitted to the card. It is used to transmit data from the reader to the card. |
The data rate of the contactless card is 106 kHz, so the length of a bit frame is 9.44 ms. A pulse in the Miller coding has a length of 3 ms. A logical “1” is expressed with a pulse in the middle of the bit frame.
Two possibilities are given to code a logical “0.” This coding depends on the previous bit. On one hand, if the previous bit was a “0,” the following “0” would be expressed with a 3-ms pulse in the first half of the next bit frame. On the other hand, if the previous bit was a “1,” the following “0” would be expressed without a pulse in the next bit frame.
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